1. Field of the Invention
The present invention relates to a manufacturing method of an airtight container. In particular, the present invention relates to a manufacturing method of a vacuum airtight container (envelope) used for a flat panel image displaying apparatus.
2. Description of the Related Art
An image displaying apparatus, in which a number of electron-emitting devices for emitting electrons according to image signals are provided on a rear plate and a fluorescent film for displaying an image by emitting light in response to irradiation of electrons is provided on a face plate, and of which the inside is maintained with a vacuum, has been known. In the image displaying apparatus like this, generally, the face plate and the rear plate are bonded to each other through a support frame, thereby forming an envelope. In case of manufacturing the image displaying apparatus like this, it is necessary to exhaust the inside of the envelope to secure a vacuum. Such an exhausting process can be achieved by several kinds of methods. As one of these methods, a method of exhausting the inside of a container through a through-hole provided on the surface of the container and thereafter sealing the through-hole by a cover member has been known.
In case of sealing the through-hole by the cover member, it is necessary to arrange a sealant around the through-hole to obtain a sealing effect. Here, several kinds of methods of arranging the sealant have been known. When one of these methods is applied to a vacuum airtight container, it is desirable to select the method which can prevent the sealant from flowing into the through-hole. This is because, although it is necessary to heat and then soften or melt the sealant to uniformly arrange and form it around the through-hole, there is a fear at this time that the sealant flows into the through-hole due to a difference between internal and external pressures of the container. In particular, in case of manufacturing the envelope of the image displaying apparatus, the sealant which has flowed inside the through-hole accounts for an electrical discharge phenomenon.
Here, Japanese Patent Application Laid-Open No. 2003-192399 (called a patent document 1 hereinafter) discloses a technique for tapering the face of a cover member opposite to a through-hole. More specifically, in the patent document 1, the distance between the tapered face and the face on which the through-hole has been formed becomes wider as the tapered face goes apart from the periphery of the through-hole. Then, a melted sealant is deformed due to the weight of the sealant itself, and the deformed sealant moves toward the tapered portion, thereby restraining the sealant from flowing into the through-hole.
U.S. Pat. No. 6,261,145 (called a patent document 2 hereinafter) discloses a technique for closing up a circular through-hole by a spherical metal cap or the like, externally filling up a sealant to the contact portion between the through-hole and the metal cap, and thus sealing the through-hole. More specifically, in the patent document 2, since the cap is fit into the tapered through-hole, the force toward the inside of a container is applied to the cap if the inside of the cap is in a vacuum. Thus, since the cap is in tight contact with the through-hole easily, it becomes difficult for the sealant to flow into the through-hole.
In the patent document 1, since the sealant directly faces the through-hole, there is a strong possibility that the sealant flows into the through-hole when it is melted. More specifically, although most sealant flows into the tapered portion, there is a possibility that a part of the sealant flows into the through-hole due to the vacuum inside the container. In the patent document 2, the sealant is applied merely to the vicinity of the cap. That is, unlike the patent document 1, the patent document 2 does not include any process of pressing the sealant. For this reason, since it is difficult in the patent document 2 to uniformly distribute the sealant, there is a possibility that it is difficult to obtain sufficient sealing performance.